Network
This section discusses network kernel parameters, including those for the following:Net coreICMPNEIGHBORCONFIGRoutingTCPIP fragmentationNet Core
Sysctl Prefix: net.core/proc Prefix: /proc/sys/net/core | Parameter | Description |
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| hot_list_length | Maximum number of skb-heads to be cached | | optmem_max | Maximum ancillary buffer size per socket | | message_burst | Limits warning messages | | message_cost | Limits warning messages | | netdev_max_backlog | Maximum number of packets queued | | rmem_default | Socket receive buffer size | | wmem_default | Socket send buffer size | | rmem_max | Maximum receive socket buffer size | | wmem_max | Maximum send socket buffer size | The message_cost and message_burst parameters are used to limit the warning messages written to the kernel log. The default setting limits warning messages to one every 5 seconds. The higher the error_cost is, the fewer messages are written. The error_bust parameter controls when messages are dropped.ICMP
Sysctl Prefix: net.ipv4/proc Prefix: /proc/sys/net/ipv4 | Parameter | Description |
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| icmp_ratemask | Mask value for ICMP types | | icmp_ratelimit | Rate for sending ICMP packets | | icmp_ignore_bogus_error_responses | Warning messages from router | | icmp_echo_ignore_broadcasts | ICMP echo requests to broadcast addresses | | icmp_echo_ignore_all | ICMP echo requests | | igmp_max_membership | Maximum number of multicast groups | The icmp_ratemask parameter contains ICMP types for which rates are limited.The icmp_ratelimit parameter limits the maximum rates for sending ICMP packets whose type matches the icmp_ratemask parameter to specific targets. A value of 0 disables any limit. The value is in jiffies.If icmp_ignore_bogus_error_responses is enabled, the kernel does not issue any log warnings when routers send bogus responses to broadcast frames.If the icmp_echo_ignore_all parameter is enabled, the kernel ignores all the ICMP echo requests sent to it. If icmp_echo_ignore_broadcasts is enabled, the kernel ignores all the ICMP echo requests sent to broadcast/multicast addresses.The igmp_max_membership parameter is the maximum number of multicast groups we can subscribe to.NEIGHBOR
Sysctl Prefix: net.ipv4.neigh.interface/proc Prefix: /proc/sys/net/neigh/interface/ | Parameter | Description |
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| locktime | Time before replacing an old ARP entry | | proxy_delay | Maximum time before answering an ARP request | | anycast_delay | Maximum delay for neighbor solicitation messages | | proxy_qlen | Maximum queue length of delayed proxy ARP | | unres_qlen | Maximum queue length for pending ARP request | | gc_stale_time | Time before a check for stale ARP entries | | delay_first_probe_time | Delay for first probe if neighbor is reachable | | base_reachable_time | Validity period of an entry once a neighbor has been found. Default is 30 seconds. | | retrans_time | Time between neighbor solicitation retransmits | | app_solicit | Number of requests to send to user-level ARP daemon | | ucast_solicit | Maximum number of retries for unicast solicitation | | mcast_solicit | Maximum number of retries for multicast solicitation | CONFIG
Sysctl Prefix: net.ipv4.conf.interface/proc Prefix: /proc/sys/net/conf/interface/ | Parameter | Description |
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| proxy_arp | Responds to ARP requests if enabled | | rp_filter | Source validation is required if enabled | | bootp_relay | Accepts packets with 0.x.y.z addresses | | accept_source_route | Accepts set of IP addresses to visit on way | | send_redirects | Sends ICMP redirects | | shared_media | Different subnet communication | | secure_redirects | Packet resend using secure interface | | accept_redirects | Packet resend on same interface on an ICMP redirect | | mc_forwarding | Multicast forwarding | | forwarding | IP forwarding | The secure_redirects parameter accepts ICMP redirect messages only for a gateway listed in the default gateway list. The send_redirects parameter determines whether such redirects are sent.Turning on the accept_source_route parameter gives a packet a list of IP addresses it should go through on its way.The bootp_relay parameter accepts packets with source addresses of 0.x.y.z with destinations not to this host as local ones. The BOOTP daemon is expected to catch and forward such packets.The mc_forwarding parameter enables or disables multicast forwarding on this interface.The shared_media parameter allows the kernel to assume that different subnets on this device can communicate directly.Routing
Sysctl Prefix: net.ipv4.route/proc Prefix: /proc/sys/net/route/ | Parameter | Description |
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| error_burst | Time before logging a warning message | | error_cost | Cost factor for logging a warning message | | max_size | Maximum size of routing cache | | max_delay | Maximum delay in seconds for flushing routing cache | | min_delay | Minimum delay in seconds for flushing routing cache | The error_cost and error_bust parameters limit the warning messages written to the kernel log. The default setting limits warning messages to one every 5 seconds. The higher error_cost is, the fewer messages are written. The error_bust parameter controls when messages are dropped.TCP
Sysctl Prefix: net.ipv4/proc Prefix: /proc/sys/net/ipv4/ | Parameter | Description |
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| tcp_app_win | Maximum window for application buffer | | tcp_reordering | Maximum reordering of packets in a TCP stream | | tcp_max_syn_backlog | Length of per-socket backlog queue | | tcp_rfc1337 | Sets TCP stack confirming to RFC1337 | | tcp_stdurg | Enables strict RFC793 interpretation of the TCP urgent pointer field | | tcp_fin_timeout | Time in seconds it takes to receive a final FIN before the socket closes | | tcp_tw_reuse | Reuse of timewait sockets | | tcp_max_orphans | Maximum number of orphaned connections | | tcp_max_tw_buckets | Maximum number of timewait sockets | | tcp_window_scaling | Enables window scaling as defined in RFC1323 | | tcp_timestamps | Enables timestamps as defined in RFC1323 | | tcp_rmem | Receive buffers for TCP socket | | tcp_wmem | Send buffers for TCP socket | | tcp_mem | Memory pages for TCP usage | The tcp_wmem parameter has three fields. The first field, min, specifies the amount of memory reserved for send buffers for a TCP socket. The second field is the default amount of memory allowed for send buffers for a TCP socket. The last field, max, is the maximum amount of memory allowed for automatically selected send buffers for a TCP socket.The tcp_rmem parameter has three fields. The first field, min, is the minimum size of receive buffer used by TCP sockets. The second is the default amount of memory allowed for receive buffers used by TCP sockets. The last field, max, is the maximum size of the receive buffer allowed for automatically selected receive buffers for TCP sockets.The tcp_mem parameter has three fields. The first field, low, specifies the low limit on the number of pages below which TCP is not bothered about its usage of memory pages. The second field is high; when the amount of memory allocated by TCP exceeds this, TCP takes corrective actions until memory consumption falls below the low value. The last field, max, is the maximum number of pages allowed for queuing by all TCP sockets. | Parameter | Description |
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| tcp_sack | Enables sack to acknowledge when a packet is received | | tcp_fack | Enables forward acknowledgment | | tcp_dsack | Enables extension to sack for duplicate packet detection | | ecn | Enables explicit congestion notification | | Parameter | Description |
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| tcp_keepalive_intvl | Frequency of keepalive probes | | tcp_keepalive_probes | Number of keepalive probes | | tcp_keepalive_time | Keepalive message interval | | tcp_syn_retries | Number of initial SYNs for outgoing connection | | tcp_retries1 | Number of retries for incoming connections | | tcp_retries2 | Number of retries before terminating | The tcp_keepalive_probes parameter is the number of times the probes are sent before killing the nonresponding connections.The tcp_syn_retries parameter is the number of times initial SYNs for a TCP connection are transmitted. This timeout is for outgoing connections. For incoming connections, the number of retransmits is defined by the tcp_retries1 parameter.IP Fragmentation
Sysctl Prefix: net.ipv4/proc Prefix: /proc/sys/net/ipv4 | Parameter | Description |
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| ip_nonlocal_bind | Binds to nonlocal IP address | | ip_no_pmtu_disc | Disables MTU path discovery | | ip_autoconfig | How host got IP configuration | | ip_default_ttl | Maximum number of hops a packet may travel | | ip_forward | Forward packets between interfaces | | ipfrag_time | Time in seconds to keep IP fragment in memory | | ip_dynaddr | Enables dynamic socket address rewriting on interface address change | | ipfrag_low_thresh | Low-memory watermark for fragment handler | | ipfrag_high_thresh | Maximum amount of memory used to reassemble IP fragments | | ip_local_port_range | Range of ports used by TCP and UDP to choose the local port | |
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